MAINTAINERS: Claim atomic*_t maintainership

Since Will and me have rewritten and heavily extended the atomic*_t
infrastructure over the past few years, claim maintainership of it.

We would also like to add Boqun as he helped out with PowerPC and has shown
good understanding of these bits.

We would still defer to architecture maintainers on implementation
details, but we'd take care of the interface and cross architecture
semantics of the primitives.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20161108140603.GH3117@twins.programming.kicks-ass.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>